It has been the common practise in the drying of clothes in a clothes drying machine for the user to set the timer controlling the drying time to a selected value which is expected to be adequate to dry the clothing placed in the dryer. As the user usually is uncertain as to the actual weight of clothing being dried, to ensure drying, an excess of time is set on the timer and the dryer operation continues longer than necessary, leading to the possibility of scorching or otherwise damaging the clothing, as well as consuming unnecessary power and adding to the drying costs. Alternately, to prevent this overdrying or to attempt to leave some residual moisture in the clothing, the timer may be set to operate the dryer for a period estimated to be less than the time required for complete drying, in which case the clothing may be inadequately dried and the dryer operation has to be cycled on and off by the user and the clothing inspected after each drying period to assess the degree of dampness thereof.
Air temperature control in larger dryers operating on a nominal line voltage of 208 or 240 v, is obtained through On-Off control of the dryer heater by means of a thermally activated switch which is arranged to maintain the air temperature in the dryer at a value to be effective for the fabric to be dried. During the "Off" periods of the heater, a timer motor is energized. Repeated application of these timer energized periods will eventually bring the timer to the end of its travel thereby terminating the drying.
The premise on which the functioning of this proposed arrangement is based, is that the supposed relation between duty cycle of the thermal switch and the clothes load is such that the frequency of the timer motor "On" periods (heater Off) decreases with increasing clothes loads. That is, the heater will have to stay on longer (and the timer will therefore be accumulating less time) to maintain the air at an appropriate temperature, say 145.degree. F, when a large mass of wet clothes is being dried than when the load is small. Therefore, the theory is that even though the timer is pre-set to the same value, the actual drying time increases with clothes load size.
Unfortunately, even in such a larger dryer having such full line voltage across the heater, the demand on the heater to raise the air temperature to the 145.degree. F, at which point it is practical to allow the heater to cycle, is such that effectively very little cycling occurs while the clothing retains any significant amount of moisture. Once the clothes load reaches in the neighborhood of 3 lbs. of clothing and above no significant drying time regulation can be achieved. Even with smaller loads, the sensitivity of the system is limited and effective regulation cannot be achieved.
In the case of smaller dryers which are required to operate on 120 v. A.C. of say 15 amperes maximum, the power supplied is totally insufficient to raise the temperature of the air in the dryer to even close to a point, e.g. an air temperature of 145.degree., where the system can tolerate cutting off the heater while the clothing retains any significant amount of moisture and therefore such heater cycling is completely impractical with respect to these small machines.